1. Field of the Invention
The present invention relates generally to the field of surface etching and cleaning. More particularly, the present invention relates to a method and apparatus for etching and cleaning surfaces, especially semiconductor substrates, using photon-enhanced neutral atomic beam surface reactions.
2. Statement of the Problem
Dry and wet etching and cleaning processes are widely used in the semiconductor industry for etching and cleaning semiconductor substrates. It is important that cleaning and etching processes do not damage the semiconductor substrate, are capable of etching anisotropically, and yet do not damage the process apparatus or micron-scale features formed on the semiconductor substrate. Also, the etching apparatus is desirably easily scaled to process many substrate sizes, including large area wafers that are 200 millimeters or more in diameter.
Plasma and wet chemical processes require production, transportation, and storage of a variety of highly reactive and toxic materials such as acids, chloro fluorocarbons, fluorine, and chlorine, for example. These materials create a personal and environmental hazard. A need exists to etch, clean, and deposit films on substrates using processes with reagent gases having reduced toxicity and reactivity and processes compatible with storage of the reactant species in a low reactivity state.
Dry etching often uses atomic oxygen plasmas to remove photoresists and clean hydrocarbon films from substrates. Atomic oxygen plasmas are formed at low pressure (about 10 Torr) and can be initiated using a direct current (DC) or alternating current (AC) field that creates oxygen ions. A serious limitation of plasma processing is that the energy of the plasma system can damage the substrate being processed. To achieve highly anisotropic etching required for modern high aspect ratio devices, the oxygen ions must be accelerated to high energy. Hence, to achieve anisotropic etching the substrate is exposed to higher levels of radiation and high frequency energy. A need exists for a method and apparatus to treat surfaces with highly directional beam for high anisotropy and high aspect ratio feature fabrication without using the high levels of radiation and high frequency energy required in plasma processing.
Although plasma processing techniques provide high quality etching and deposition, they require a large amount of process gases. In fact, only a small percentage of these process gases are activated (i.e., brought to an energy state sufficient to chemically react) and participate in useful chemical reactions. Hence, a large portion of the process gases is wasted. Even worse, a large portion of the process gases actually etch and damage the reaction chamber containing the plasma, requiring frequent maintenance. A need exists for an etching, cleaning and deposition technology that reduces the gas load of the reactor to use less of the reactant gases and produce less toxic waste while simplifying the processing apparatus. Similarly, a need exists to reduce exposure of the apparatus itself to the harmful effects of plasmas, radiation, and highly reactive ions.
Another side effect of plasma processing is that the substrate that is being processed is often exposed to the high temperature and radiation of the plasma. Semiconductor devices with fine geometry features can easily be damaged by the temperatures and radiation. For example, it is well known that thin gate oxides can be ruptured or permanently damaged by hot electron injection into the oxide during plasma etching. Also, because the plasma creates a wide variety of reactive species and ions due to the variety of reactions that take place, the surface of the substrate is exposed to all of the reactive species. Often, some of these reactive species create unpredictable and undesirable process variations. Again using the gate oxide structure, it is known that hydrogen ions can be tapped in thin oxide films during plasma etching or cleaning thereby creating unpredictable electrical performance. Further, high temperatures severely limit the kinds of processes which are available. Substrates having metal structures, polyamide structures, or soft glass structures cannot be exposed to high temperatures. A need exists to treat surfaces with reactant neutral atoms that avoids or minimizes the substrates' exposure to high temperatures, radiation, and plasma fields.
An apparatus for producing a single beam of atomic oxygen is described in a paper entitled "New Molecular--Dissociation Furnace for H & O Atom Sources" by Bert Van Zyl and M. W. Gealy published Nov. 10, 1985 in Review of Scientific Instruments 57, (3). This article describes a molecular dissociation furnace which produces a single atomic beam using an electron bombarded furnace tube. While the single beam apparatus was useful in research studies on atomic beams, it was inapplicable to commercial applications requiring treatment of a large surface area.
U.S. Pat. No. 4,662,977 issued May 5, 1987 to Motley et al. describes a neutral beam processing apparatus that uses an ionizing plasma to create ionic reactants and form them into a beam and then neutralizes the ion beam. Like the apparatus described in the Van Zyl/Gealy article, this atomic beam is difficult to scale and suffers many of the difficulties of plasma processing because plasma fields are involved.
U.S. Pat. No. 4,780,608 issued to Cross et al. on Oct. 25, 1988 illustrates an atomic beam apparatus using a sustained laser discharge to dissociate oxygen molecules into oxygen atoms. The neutral oxygen atoms are formed into a beam; however, the beam apparatus is not easily scaled for processing large diameter substrates.
U.S. Pat. No. 5,188,671 issued to Zinck et al. on Feb. 23, 1993 and U.S. Pat. No. 4,901,667 issued to Suzuki et al. on Feb. 20, 1990 describe molecular beam apparatus for forming large and small area molecular beams. Molecular beams can be formed at lower temperature than atomic beams since there is no need to dissociate the molecules into atoms. Molecular beams thus require either more reactive chemicals or subsequent plasma or thermal enhancement to cause reactions at the substrate.
U.S. Pat. No. 4,920,094 issued to Nagawa et al. on Apr. 24, 1990 illustrates a superconducting thin film deposition apparatus that uses neutral beams in a sputtering apparatus. The neutral oxygen beam is formed by oxygen ionization and subsequent neutralization and so suffers the difficulties of any plasma processing system.
U.S. Pat. No. 5,284,544 issued to Mizutani et al. on Feb. 8, 1994 describes a neutral beam generating apparatus that creates an ion beam using a plasma reactor that neutralizes the ion beam before treating a surface. The Mizutani method uses a microwave plasma generator and combines a neutral beam with a radical supply source from the plasma to encourage surface treatment. Ions are prevented from reaching the surface by an ion screen or grid; the latter, however, is subject to sputtering and erosion, which can contaminate the surface being treated.
3. Solution to the Problem
The above-identified problems are solved by a method and apparatus that generates a plurality of low-energy reactive neutral beams at a semiconductor substrate. The neutral reactive beams are generated by dissociation and are directed to the semiconductor substrate to etch or clean without electrical or radiation damage associated with plasma processing. Because any number of beams may be used, the process and apparatus are scaleable to any substrate size. The neutral reactive beams are highly directional even at low energy, and so avoid physical damage to the substrate. Because the beam is highly directional and can be fairly uniform chemically, a large portion of the generated beam participates in desired reactions thereby minimizing the gas load on the processing apparatus and minimizing waste gas production. Also, the reactant species can often be stored as a low reactivity molecule in the preferred embodiment, minimizing the hazards of production, transportation, and storage of the reagent chemicals.